1. Field of the Invention
The present invention generally relates to the art of wireless mobile communication system. In particular, the present invention relates to the art of searching for the pilot signals of base stations to establish communication between a mobile unit and the base station.
2. Description of Related Art
In wireless communications technology, user data (e.g. speech) is encoded in a radio frequency for transmission and reception between a base station and a mobile unit. The radio spectrum allocated by regulatory authorities for a wireless system is "trunked" to allow simultaneous use of that spectrum block by multiple units.
The most common form of trunked access is the frequency-division multiple access (FDMA) system. In an FDMA system voice is commonly transmitted through analog modulation but can in principle be digitized and transmitted with digital modulation. In FDMA, the spectrum is divided into frequency channels comprised of distinct portions of the spectrum. The limited frequency channels are allocated to users as needed. However, once a frequency channel is assigned to a user, that frequency channel is used exclusively by the user until the user no longer needs the channel. This limits the number of concurrent users of each frequency channel to one, and the total number of users of the entire system, at any instant, to the number of available frequency channels.
Another common trunking system is the time-division multiple access (TDMA) system. TDMA is commonly used in telephone networks, especially in cellular telephone systems, in combination with an FDMA structure. In TDMA, data (speech) is digitized and compressed to eliminate redundancy thus decreasing the average amount of bits required to be transmitted and received for the same amount of information. The timeline of each of the frequency channels used by the TDMA system is divided into "frames" and each of the users sharing the common channel is assigned a time slot within the frames. Each user then transmits a burst of data during its assigned timeslot and transmits nothing during other times. With the exception of delays required by the bursty data transmission, the TDMA system will appear to each of the users sharing the frequency channel to have provided an entire channel to each user.
The FDMA and TDMA combination technique is used by the GSM (global system for mobile communications) digital cellular system. In GSM, each channel is divided up in time into frames during which eight different users share the channel. A GSM time slot is only 577 .mu.s (micro-seconds), and each users gets to use the channel for 577 .mu.s every 4.615 ms (milli-seconds). 577 .mu.s*8=4.615 ms.
Yet another method for sharing a common channel between multiple users is the code-division multiple access (CDMA) technique using direct sequence spread spectrum modulation. CDMA is relatively new to the cellular technology and is one of the accepted techniques to be included into the next generation of digital cellular systems in the United States of America (U.S.A.).
As with TDMA, the CDMA systems are typically used in conjunction with a FDMA structure, although this is not required. However, unlike the TDMA system, the CDMA system does not separate the multiple users of a common frequency channel using time slices. Rather, in CDMA, multiple users are separated from each other by superimposing a user-specific high-speed code on the modulation of the data of each user. Because the separating code has the effect of spreading the shared channel of each user's transmission, the CDMA system is often called a "spread spectrum" system.
"Direct sequence" spreading is accomplished by multiplying a narrowband information carrying signal by a much wider band spreading signal. The error coded and digitally modulated data (speech) for each of the shared users of the CDMA channel may typically be 9.6, 14.4, or 19.2 KHz wide. This is spread using a much wider spreading signal which may be 1.2288 MHZ wide. Using the wider spreading signal, a CDMA frequency channel can accommodate many users-on code sub-channels. The spreading signal is usually a sequence of pseudo random bits (PN code) and is often called a "spreading code," or "chipping code" because it "spreads" or "chips" the much slower data bits. The PN code is different for differing users, allowing a user to distinguish its code sub-channel from other users' sub-channels on the same frequency channel. The PN sequence may be expressed as c(t), where the chipping function, c(), is a function of time t. The PN sequence is generated using a linear feedback shift register (LFSR) which outputs a random-like sequence of digital ones and zeros. These digital ones and zeros are modulated to -1 and +1 respectively and filtered to give the chipping function c(t). Thus the chipping function has the property that c(t).sup.2 =+1. The PN sequence generated by a N-register LFSR is 2.sub.N -1 chips long, though a common system artificially inserts a zero to extend the full sequence length to 2.sup.15 =32768 chips. That system has a chip rate of 1.2288 MHz, so that the sequence repeats every 26.666 ms.
In a typical system, each base station maintains a pilot channel with its own identifying spreading code for the mobile units to refer to. A pilot signal is a spread signal with no underlying information modulation, such that the exact waveform is known by both transmitter and receiver, with the exception of the waveform timing. The mobile units use the pilot channel to synchronize themselves with the base station so they can effectively communicate with the base station. When a mobile unit is powered on, the mobile unit initially searches for a pilot channel in an attempt to establish a lock with a base station. This process is called "acquisition." In order to "acquire," or lock on, to a base station, the mobile must align its locally generated version of the PN sequence with the PN sequence of the base station by determiing the timing of the transmitted pilot's spreading sequence. The present invention provides for an improved acquisition technique.
At power up, a mobile unit must search for a pilot to synchronize its spreading sequence with that of a base station. The acquisition process is generally described using FIG. 1. FIG. 1 is a simplified diagram illustrating the major functions of the acquisition process.
In the simplified model of FIG. 1, the radio signal is received by an antenna 12. The signal at line 14 is a radio frequency signal which is about 800 to 900 MHz for cellular communications. The signal at line 14, S.sub.14, can be expressed as EQU S.sub.14 =d(t)c(t-D.sub.base)cos(t)
where
d(t) is the data (speech in digitized form); PA1 c(t-D.sub.base) is the PN short code at delay D.sub.base which is the base station delay; and PA1 cos(t) is the radio frequency carrier wave. PA1 1. The incoming signal is multiplied by a multiplier 24 by a PN code with an initial delay, D.sub.test, 44. PA1 2. The result of the multiplication is summed, or accumulated 28, for N number of chips, N being a predetermined number of chips. The pilot signal could be thought of as being constructed based on a sequence of zero's (0) and one's (1). It is common in the industry to refer to each digit of a digital spreading sequence as a "chip." For example, a digital spreading signal of a fixed duration containing 100 digital values can be called a set of 100 chips. PA1 3. The energy of the accumulated value of the products are calculated 34 by taking a magnitude squared of the accumulant. PA1 4. The calculated energy is compared to some pre-set threshold, .gamma., 38. PA1 5. And, a determination is made. If the calculated energy equals or exceeds the threshold value, .gamma., then the given delay being tested, D.sub.test, 44 is determined to be a potential signal and is verified. If the verification is successful, then D.sub.test is determined to equal D.sub.base and the acquisition terminates. If the calculated energy is less than the threshold value, then D.sub.test 44 is not equal to D.sub.base, and the next delay value is tested beginning at step 1. In fact, the delay value is tested at every 1/2 chip. Therefore, the number of delays tested is 2 * 2.sup.15, or 2.sup.16.
Of course, c(t-D.sub.base) is the spreading code sequence used in the CDMA system, and would not be present in a non-CDMA system. A pilot signal contains no data, so in the case of a pilot signal d(t)=1 and is constant. The pilot signal spreading code is a different PN code from the data spreading code, allowing the two signals to be distiguished. Once the pilot code timing is known, that same timing can be applied to the data spreading code to allow the receiver to demodulate the digital data.
The process of acquisition, then, is the process of determining the value of D.sub.base. Once the value of D.sub.base is determined, the mobile can use the same c(t-D.sub.base) sequence to lock on to the base signal and remove the spreading code to retrieve the data, d(t).
The quadrature demodulator circuit 16 removes the carrier wave portion, cos(t), from the incoming RF signal and provides a complex valued baseband signal to the sampling circuit 20 which converts the analog RF into digital samples at the spread spectrum frequency of 1.2288 MHZ. At line 22, the signal can be expressed as EQU S.sub.22 =d(t)c(t-D.sub.base)
The base station delay, D.sub.base, is not known by the mobile unit at power up. If D.sub.base is known, then the PN code delay at the mobile unit, D.sub.mobile, can be set to match D.sub.base, and S.sub.22 can be multiplied by c(t-D.sub.mobile) to eliminate the spreading sequence to retrieve the data. Alternatively expressed, if D.sub.mobile =D.sub.base, then EQU d(t)c(t-D.sub.base)c(t-D.sub.mobile) =d(t)c(t-D.sub.base)c(t-D.sub.base) =d(t); because c(t).sup.2 =1
Fixed Dwell Search System (FDSS)
Assuming that N=15 such that the full sequence length is 2.sup.15, at power up, D.sub.base is not known, and the mobile must test each of the 2.sup.15 possibilities to find D.sub.base. In the Fixed Dwell Serial Search (FDSS) systems, D.sub.base is found by brute-force, trial and error method which can be outlined as follows (continuing to refer to FIG. 1):
The multiplication (step 1 above) and the summation (step 2) are typically done using a specially designed hardware, and is performed at the same speed as the incoming chip rate. The energy calculation (step 3) and the comparison with a threshold (step 4) could be performed in software by a digital signal processor (DSP) 42 as indicated by the dash line in FIG. 1
If the incoming signal at line 22, S.sub.22, is multiplied by the correctly delayed PN code, then the sum, or integration, of the energy levels of a set of chips will add up to a signal strength approaching some amplitude value, A. If the incoming signal at line 22, S.sub.22, is multiplied 24 by an incorrectly delayed PN code 44, then the signal at line 26, S.sub.26, will appear as noise and the sum 28, or integration, of the energy levels of the set of chips will approach zero. The DSP 42, controls the value of D 44, to increment the phase based on the dwell/threshold decisions. This "de-spreading" method is discussed in many text and reference books. For example, see Redl, et. al., AN INTRODUCTION TO GSM, pp. 61-63. In reality, however, the results of the integration do not fall exactly at A or exactly at 0, but are corrupted by noise and fall near A or near 0, and appear as some probability function near A or 0.
In addition to the above method of increasing the number of concurrent users of a cellular communication system, the Space Division Multiple Access (SDMA) is almost universally used to increase the number of concurrent users of the system. As its name implies, the SDMA systems operate based on the fact that geographical regions, or spaces, can be logically divided up into cells. Then, the same channel can be reused in differing cells.
In SDMA, communications within each cell is typically handled by that cell's base station; however, when a user is attempting to acquire a base station, the user may lock on to a base station of an adjacent cell. The reason for the existence of this problem is the fact that the above described acquisition process locks on to the first pilot signal that the mobile unit detects instead of the strongest signal.
This problem can be illustrated by FIG. 2. When the mobile cellular telephone user 50 turns on a mobile telephone unit, the unit initiates the above-described acquisition process. Even though the user 50 is located in the cell 52 having base station 62, the user 50 may acquire base station 64, 66, or 68, located in cell 54, 56, and 58, respectively. If any of those base stations has a pilot signal the energy of which is (1) greater than the threshold, and (2) the PN code delay, or the code phase, of which is tested by the mobile unit before the code phase of base station 62, then the mobile unit will lock on that base station rather to base station 62 Even though the base station 62 may have a stronger signal.
The user's acquisition of a out-of-cell base station leads to inefficient allocation of channels and degrades the performance of the in-cell base station 62. Moreover, transmission of signal to station 64 requires too much signal strength from the mobile user, drowning out the closer station 62.